Conventional fins can be generally divided into two types, namely, a fin 10 with closed foot pocket 40, as shown in FIG. 1, and a fin 20 with open foot pocket 40, as shown in FIG. 2. In both types, the foot pocket 40 is integrally connected to a blade 30 of the fin 10, 20. The closed foot pocket 40 of the fin 10 has a front portion 41 that is completely connected to the blade 30 without any opening between them and therefore encloses toes of a diver wearing the fin 10 in the foot pocket 40. On the other hand, the open foot pocket 40 of the fin 20 has a front opening 42 between the foot pocket 40 and the blade 30 to expose the diver's toes.
As known by most people, in skin diving or other types of diving, the diver moves forward under water when he or her alternately swings two legs up and down to kick water. The diver's strength is transferred from thighs via shanks to insteps and toes, enabling insteps and toes to force against the foot pockets 40 and thereby bring the blades 30 to downward move against water. Please refer to FIG. 3. For a diver wearing fins 10 with closed foot pockets 40, when one of his or her two legs is moved downward to kick water, the leg's strength is transferred to toes 43, forcing the same to press against the foot pocket 40 at an inner side of the front portion 41 thereof and bring the blade 30 to downward move against water. It is known that toes are the most vulnerable portions in human's feet. Improperly or overly applying a force with toes tends to cause tiredness and pain of feet. When the toes 43 forcefully press against the closed foot pocket 40, the diver's sole arches to easily cause sprained toe joints as well as cramped foot and calf.
Please now refer to FIG. 4. The open foot pocket 40 of the fin 20 allows toes 43 to expose from the front opening 42 thereof. When the diver wears fins 20 with open foot pockets 40, his or her leg strength of the downward moved leg is transferred to an instep 44 of that leg, making the instep 44 a point of applying force. The instep 44 applies a force on the foot pocket 40 that transfers the force to the blade 30 to complete the movement of kicking water. Since the instep 44 is structurally stronger than toes 43, it is more advantageous to move the blade 30 with the instep 44 than toes 43. The fin 20 with open foot pocket has, however, a drawback of dangerously exposing toes 43 and a part of the instep 44 from the front opening 42 of the foot pocket 40 to rocks, coral reefs, sea urchins and the like under sea, resulting in cut wound and/or puncture wound at these areas. Moreover, the front opening 42 of the open foot pocket 40 has a hard and sharp front edge 45 that would be inevitably in contact with and abrade or otherwise compress the instep 44 at some fixed areas thereof when the latter exerts force against the foot pocket 40, resulting in swollen, blistered, bruised skin and/or soreness and stiffness of the instep 44. Such harms made by the open foot pockets 40 to the diver's insteps prevent the diver from kicking water with full strength and make diving less interesting.